Reduced rolling resistance pneumatic radial tire and method of manufacturing the same

ABSTRACT

A pneumatic radial tire is adapted to improve or reduce the rolling resistance and simultaneously to improve the steering stability and riding comfortability against vibration during running without adversely affecting the wet performance and durability by making higher the carcass maximum width position and the carcass curvature reverse position, or enlarging the curvature of the carcass in the radially outer zone of the carcass maximum width position and simultaneously the carcass curvature reverse extent in the radially inner zone of the sidewalls so as to intentionally deviate from the naturally equilibrated configuration. 
     A method of manufacturing a reduced rolling resistance pneumatic radial tire uses a mold for vulcanizing and forming the tire therein, which mold has a particular shape of cavity having a foot width 20-50% wider than a rim width and diverging progressively from shoulder corresponding portions to the wider foot portion of the cavity.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a pneumatic radial tire having low rollingresistance and a method of manufacturing the same. More particularlythis invention relates to a tire adapted to improve various tireperformances, inter alia to effectively reduce the rolling resistance byproperly modifying a radial surface profile of a carcass.

2. Description of the Prior Art

It is known that in order to reduce the rolling resistance of a tire itis necessary to reduce the energy which is to be consumed due toperiodic stresses and strains caused in the tire while rolling. It is areasonable assumption that rates of consumed energies at respectiveportions of tires are approximately 34% at treads, 27% at buttressportions, 25% at sidewalls and 14% at bead portions according to aresult of analysis on percentages of consumed energies of so-calledradial carcass tires whose practical usefulness has been recognizedunder normal used conditions that is, inflated at internal pressure ofthe order of at the most 2 kg/cm².

The tread has the largest contribution to the rolling resistance of thetire, and in order to reduce inner friction and hence the rollingresistance, therefore, the compounding ratio of tread rubber has beengenerally selected so as to increase its resilience. In this case,however, wet performance, which is one of the important performances ofthe tire, is adversely affected undesirably depending upon the degree ofthe reduced rolling resistance.

With the above solution, accordingly, the rolling resistance cannot begreatly decreased unless a particular precaution is taken to prevent thewet performance from being lowered. Since an effective method tomaintain the wet performance has not been found yet, the above solutiondoes not achieve a significant effect.

It has been further proposed to use for sidewalls the rubber compoundingratio having reduced inner friction in the same manner as in the tread.This proposal, however, serves only to reduce 3% or less of the rollingresistance but adversely affects its damping capacity for vibrationcaused in the tire and hence its important riding comfortability.

Moreover, it has been proposed to change two layer carcasses into singlelayer carcasses or to narrow belt widths to make light weight tires inorder to reduce the rolling resistance of the tire. However, suchproposals unavoidably reduce the rigidity of important portions of thetires to decrease the steering stability, so that only limited effectscan be expected.

SUMMARY OF THE INVENTION

In contradistinction to the above solutions of the prior art, theinvention of the application resides in the discovery that by suitablymodifying the carcass radial surface profile the rolling resistance of atire can be remarkably reduced and simultaneously the steering stabilityand the riding comfortability against vibration can also be improvedwithout adversely affecting the wet performance and durability of thetire. This discovery occurred as a result of the inventor's fundamentalinvestigation on deformations of sidewalls of rolling tires subjected toloads.

It is an object of the invention to provide an improved pneumatic tirewhose rolling resistance is remarkably reduced with improved steeringstability and riding comfortability without adversely affecting its wetperformance and durability by making higher the carcass maximum widthposition and the carcass curvature reverse position, or enlarging thecurvature of the carcass in the radially outer zone of the carcassmaximum width position and simultaneously the carcass curvature reverseextent in the radially inner zone of the sidewalls so as tointentionally deviate from the naturally equilibrated configuration.

It is another object of the invention to provide a method ofmanufacturing a pneumatic tire by vulcanizing and forming in a moldcomprising a cavity having a foot width 20-50% wider than a rim width toobtain the tire with reduced rolling resistance and improved steeringstability and riding comfortability without sacrificing its wetperformance and durability.

In order that the invention may be more clearly understood, preferredembodiments will be described, by way of example, with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration for explaining a relation between aradial surface profile of a tire in a solid line according to theinvention and an arc of a standard circle in a broken line;

FIG. 2 is a schematic illustration for explaining deformation when atire is subjected to a load;

FIG. 3a is a schematic sectional view of the tire according to theinvention;

FIG. 3b illustrates a comparison of the carcass radial surface profileof the tire shown in FIG. 3a with a naturally equilibrated carcassprofile;

FIG. 4a is a schematic sectional view of the tire of a furtherembodiment of the invention;

FIG. 4b illustrates a comparison of the carcass radial surface profileof the tire shown in FIG. 4a with the naturally equilibrated carcassprofile and a standard circle;

FIG. 5 is a schematic illustration for explaining the deformation of atire having a naturally equilibrated profile when filled with innerpressure;

FIG. 6 is a schematic illustration for explaining the deformation of thetire having the carcass radial surface profile according to theinvention when filled with inner pressure;

FIG. 7 schematically illustrates various cavity profiles for explainingthe mold cavity according to the invention;

FIG. 8 schematically illustrates carcass radial surface profiles ofcomparative examples and according to the invention; and

FIG. 9 schematically illustrates carcass radial surface profiles ofcomparative examples and according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In general, it has been known that the deformation of sidewalls isdivided into bending deformation and shearing deformation. The inventionresides in a discovery that radially inner portions of sidewalls (in theproximity of bead portions) are subjected to the bending deformationmuch more than shearing deformation but the radially outer portions ofthe sidewalls (in the proximity of a tread) are subjected to shearingdeformation much more than the bending deformation.

The shearing deformation at the radially outer portions of the sidewallsis more complicated than the bending deformation. The inventors of thepresent application have carefully investigated these deformations tofind the following important points.

First, during rolling under a load the shearing deformation in theradially outer zones of the sidewalls of a tire is relatively smallimmediately below the load but considerably large in zones which areabout to be in contact with and away from a ground surface, which takesa large part of the entire consumed energy.

Second, the shearing and bending deformations are in a reciprocalrelationship where when one increases, the other decreases.

In view of the above fact, it has been found that even if the bendingdeformation concentrated immediately below the load in the radiallyouter portions of the sidewalls being comparatively thin increasessomewhat, entire consumed energy can be reduced by decreasing theshearing deformation playing a large part in the consumed energy.

In order to increase the bending deformation in the radially outer zonesof the sidewalls, it is necessary to enlarge a curvature of profilelines of shoulders of a carcass corresponding to the radially outerzones of the sidewalls when the tire mounted on a proper rim is filledwith normal inner pressure. The inventors have studied radii R of thecurvatures of the profile lines in various manner to find that a radiusR whose ratio R/R' to a radius R' of standard circle shown in FIG. 1 iswithin 0.65-0.85 is effective for the above purpose, which will beexplained in more detail later.

It should be noticed in this case that the arc or curved line BEC shownin FIG. 1 is a mere reference or standard arc and is inherentlydifferent from a carcass radial surface profile under the naturallyequilibrated condition. However, the radially outer portions of thesidewalls are relatively thin and have relatively low rigidity, so thatthe profile of the carcass radial surface under the naturallyequilibrated condition substantially approximates the part of the arc ECof the circle. It should be therefore noticed that the above value0.65-0.85 of the ratio R/R' will be obtained only insofar theconfiguration under the naturally equilibrated condition isintentionally precluded.

When the ratio R/R' is more than 0.85, it is impossible to obtain theeffect for reducing the rolling resistance by reducing the shearingdeformation in the radially outer portions of the sidewalls byintentionally precluding the configuration under the naturallyequilibrated condition.

When the ratio R/R' is less than 0.65, the bending deformation isconcentrated in relatively thick buttress portions to cancel the effectfor reducing the rolling resistance resulting from the reduced shearingdeformation.

Then, the consumed energy due to the bending deformation at the radiallyinner zones of the sidewalls is generally indicated by the followingequation.

    Consumed energy=A.E. tan δ.(ΔC).sup.2.S        (1)

where

A: a suitable constant

E: modulus of elasticity of radially inner portion of sidewall

tan δ: loss tangent

ΔC: variation in curvature of radially inner portion of sidewall bybending deformation, and

S: length of radially inner portion of sidewall from the widest portionof carcass

It is clear from equation (1) that if the E, tan δ and S arerespectively the same, the consumed energy is proportional to (ΔC)². Theinventors have investigated the variation in curvature of the radiallyinner portions of sidewalls of actual tires subjected to load to find animportant fact that the tire has at its radially inner portion ofsidewalls an outwardly extending curvature of radius R₁ beforedeformation, but the sidewalls are reversely deformed to have aninwardly extending curvature of R₂ after loading. This is shown in FIG.2.

The change (ΔC)² in curvature at the radially inner portions of thesidewalls in the equation (1) is then indicated as follows. ##EQU1## Ifthe tire mounted on a rim and filled with normal inner pressure hasalready at the radially inner portions of sidewalls inwardly extendingcurvatures, the change (ΔC')² in curvature is indicated as follows.##EQU2## From equations (2) and (3) it is clear that (ΔC')² is less than(ΔC)² and the consumed energy according to the equation (1) becomescorrespondingly small.

From this viewpoint, it is effective to provide an inwardly extendingcurvature at the radially inner portions of the sidewalls when the tireis filled with the normal inner pressure. In consideration of the factthat the bending deformation is primarily caused at the carcass mainlysupporting the filled inner pressure, the height of the reversingposition of the curvatures (shown as D in FIG. 2) is most desirable tobe as high as possible to suitably cause the inwardly extendingcurvature at the carcass located at the radially inner zones of thesidewalls.

The height D is needed to be equal to the height at a location where theradially inward zones of the sidewalls are reversely deformed from theoutward to inward direction. Although this height is dependent upon theconstruction in the radially inward portions of the sidewalls, theheight is needed to be in the range of 20-35% of the height H of thetire mounted on a rim and filled with the normal inner pressure.

As above mentioned, in order to increase the bending deformation in theradially outer zones of the sidewalls, it is necessary to enlarge thecurvature of profile lines of shoulders of the carcass when the tiremounted on the proper rim is filled with normal inner pressure. For thispurpose, in addition to the height D, it is necessary to properly selectthe height (shown by E in FIG. 2) of the location where the carcass isthe widest in the upper portions of the sidewalls.

The inventors have investigated the height E to find it to be within arange of 50-65% of the height H of the tire mounted on the rim andfilled with the normal inner pressure.

In this case, if the value of E exceeds 65% of the H, the bendingdeformation is concentrated at the relatively thick buttress portions tocancel the effect resulting from the reduced shearing deformation. Ifthe value of E is less than 50% of the H, a very unnatural carcassradial surface profile is formed in conjunction with the height D of thecarcass curvature reverse position to adversely affect the tiredurability and to make difficult the manufacturing of the tire.

As above mentioned in detail, the present invention deals with a tireincluding at least one layer of carcass 3 consisting of a ply made ofrubber coated organic fiber cords arranged substantially in radialsurfaces of the tire and extending about bead wires 1 and returningtoward radially outwardly of the tire embracing hard rubber fillers 2therebetween, at least two layers of belts 4 of rubber coated cordshaving a high modulus of elasticity and intersecting each other atrelatively slight angles for reinforcing the tire in conjunction withthe carcass 3, and rubbers of sidewalls 5 on both sides of the carcass 3and a rubber of a tread 6 outside the belts 4. This is shown in FIG. 3a.When the tire is mounted on a rim 7 and filled with the normal innerpressure, according to the invention, the height E at the location Qwhere the carcass 3 is the widest is within the range of 50-65% of theheight H of the tire and the height D at the location P where thecarcass 3 is deformed from the outward to inward direction is within therange of 20-35% of the height H of the tire. It is necessary to form thetire in a mold so that when the carcass 3 is firmly held as formed andvulcanized and the tire is mounted on the rim and filled with an innerpressure corresponding to 5% of the normal inner pressure, the carcass 3in the radially outer zone has a radius R_(i) of curvature having itscenter located inside the carcass 3 between a crown 6 and the location Qof the maximum carcass width, while the carcass 3 in the radially innerzone smoothly continues with the above outwardly extending carcassportion and has a radius R_(o) of curvature having its center locatedoutside the carcass 3 as shown in a phantom line in FIG. 3a. We haveonly explained and shown the left half of the tire in FIG. 3a because itis symmetrical with respect to its equatorial line O--O. The same holdstrue in further explanation and other drawings.

As can be seen from FIG. 3b in comparison of the profile of the carcassaccording to the invention with the naturally equilibrated carcassradial surface profile (in a broken line in FIG. 3b), it should benoticed that when the tire is mounted on a rim and filled with thenormal inner pressure, the profile according to the invention is alreadybetter approximated to the deformed profile subjected to a load than inthe tire of the prior art. It should be easily understood therefore thatthe tire according to the invention can decrease the consumed energy dueto periodic variation in stress and strain caused by the tire rollingwith the aid of the appropriate variation of the carcass radial surfaceprofile when subjected to the load.

Furthermore, the deformation f (FIG. 3a) of the carcass at the radiallyinner zone when the tire is filled with the normal inner pressure ispreferably within a range of 5-10% of the height D at the carcassreverse position.

Organic fiber cords may be used for the carcass 3 such as polyester,nylon, rayon or aromatic polyamide fiber (Kevlar). Metal cords,typically steel cords, organic fiber cords such as rayon, polyester andaromatic polyamide fiber cords and unextensible cords such as glassfiber may be used for the belt 4. These cords are arranged obliquely atslight angles such as 10°-25° with respect to the equatorial line andsuch a plurality of layers are arranged one above the other to locatetheir cords intersecting to each other.

In superimposing the belts 4, the respective belt layers may be used asboth their edges are cut or auxiliary plies may be used which consist ofone or more layers having thermal contractible, for example, nylon cordsarranged in substantially parallel with the equatorial line of the tireto cover the edges of the belts. Moreover, at least one of the beltlayers may be provided at its both edges with turned portions locatedinside or outside the other belt or enclosing the cut edges of the otherbelt.

The rubber filler 2 arranged between the radially innermost portion andradially outwardly returning portion of the carcass 3 is radiallyoutwardly tapered from the top of the bead wire 1 to a location Fslightly beyond the height D of the curvature reverse position. Therubber filler has a rubber hardness of 80°-97° in Shore hardness A andphysical properties of a loss tangent 0.15-0.25 and a loss modulus 8×10⁷-2.5×10⁸ dyn/cm².

The end of the radially outwardly returning portion of the carcassextends preferably to a location of a height h which is beyond theradially outer end of the hard rubber filler but radially inward of thecarcass maximum width position Q.

Moreover, the inventors have investigated the degree of the reversingthe curvature of the carcass to find that the degree can be measured bythe maximum distance f between the carcass line FB and arc BE as shownin FIG. 1 and its suitable range is 5-10 mm.

As above mentioned, the arc BEC is a mere standard circle but isdifferent from the carcass radial surface profile based on the naturalequilibrated configuration. Since the radially inner portions of thesidewalls have the relatively high rigidity because the carcass extendsabout the bead core to turn up radially outwardly, between which arubber filler is located to reinforce the bead portion, the carcassradial surface profile based on the naturally equilibrated configurationis generally positioned inside of the arc BE.

In contrast herewith, the value f of 5-10 mm is obtained only byreversing the curvature of the carcass in the radially inner portions ofthe sidewalls by intentionally precluding the natural equilibriumconfiguration and completely different from such equilibratedconfiguration. If the value of f is less than 5 mm, the effect forreducing the consumed energy according to the principle of the equations(1)-(3) cannot be sufficiently achieved. On the other hand, if the valueof f exceeds 10 mm, the tension in the carcass in the radially innerportions of the sidewalls when filled with the inner pressure toadversely affect the durability of the tire, and as the carcass entersinside of the tire, outer surfaces of the tire position on relativelyinner sides to adversely affect the fitting of the rim.

According to a further embodiment of the invention shown in FIG. 4a,therefore, when the tire is mounted on a rim 7 and filled with thenormal inner pressure, a radial surface profile of the carcass isselected so that R/R' is within a range of 0.65-0.85 and f is within arange of 5-10 mm to reduce the rolling resistance of the tire, where: Ais a point where a flange of the rim 7 starts leaving the outer surfaceof the tire, B is an intersection of the carcass 3 and a line extendingthrough the point A in parallel with a rotating axis of the tire towardthe inside of the tire, C is an intersection of the carcass 3 and a lineextending through the point B and radially perpendicular to the rotatingaxis of the tire, D is a middle point of a line segment BC, E is anintersection of a line passing through the point D and in parallel withthe rotating axis of the tire and a line passing through the maximumcarcass width point F and perpendicular to the rotating axis of thetire, G is an intersection of the line segment BC and a line passingthrough the point F and in parallel with the rotating axis of the tire,and I is an intersection of the carcass 3 and a line passing through amiddle point H of a line segment GC and in parallel with the rotatingaxis of the tire, R is a radius of a circle passing through the pointsF, I and C, R' is a radius of a circle passing through the points B, Eand C and f is the maximum distance between the arc BE and the partialarc FB of the carcass 3 in the radial direction of the tire.

As can be seen from FIG. 4b in the same manner as FIG. 3b, the profileaccording to the embodiment of the invention has already moreapproximated to the deformed profile subjected to a load and thereforethe tire according to this embodiment can also decrease the consumedenergy with the aid of the appropriate variation of the carcass radialsurface profile when subjected to the load.

In the embodiment shown in FIGS. 3a and 3b, the materials of the cordsfor the carcass 3 and materials and arrangement and constitution of thecords for the belt 4 have been described. The same holds true in thisembodiment shown in FIGS. 4a and 4b. The tire to which this invention isapplied is of course a general radial tire having no particularreinforcing rubber layer at sidewalls, such as non-puncture tires.

In order to maintain the reverse curvatures of the carcasses and providethe value of f of 5-10 mm, it is necessary to give a large bendingrigidity to the proximity of bead portions. However, the reinforcementin the proximity of the bead portion with excess reinforcing memberstend to adversely affect the decrease of rolling resistance aimed in thepresent invention. Accordingly, a suitable selection of the componentmembers is important to achieve the object of the invention. As abovementioned, it is one selection to make at least part of the bead fillerrubber of a hard rubber stock having a hardness of 80°-97° in Shore Ahardness. If the hardness is less than 80°, the effectiveness cannot besignificantly accomplished. If the hardness is more than 97°, it givesrise to a disadvantage in durability although it is advantageous formaintaining the carcass radial surface profile.

As mentioned above, the tire according to the invention includes thecarcass radial surface profiles precluding the naturally equilibriumconfiguration of the carcass. This fact can be easily recognized byobserving the variation in carcass radial surface profile when the tireis being filled with inner pressure.

FIGS. 5 and 6 illustrate the variation in carcass profile of tires of185/70 SR14 each mounted on a rim being filled with the inner pressurefrom 5% of a nominal pressure to the normal inner pressure. In case of aso-called naturally equilibrated profile, as shown in FIG. 5, an entiresidewall bulges uniformly. In contrast herewith, with the carcass radialsurface profile according to the invention, the bulged deformation f inthe zone radially inward of the carcass maximum width position is muchgreater than that in the zone radially outward of the carcass maximumwidth position which is substantially unnoticeable as shown in FIG. 6.The solid and broken lines in FIGS. 5 and 6 are obtained by reproducingthe inner profiles of the carcass radial surfaces by the use of plasterbefore and after the bulged deformation.

The difference in bulged deformation between the invention and the priorart will of course affect the distribution of tension in the carcasses.With the tire according to the invention, the carcass in the proximityof the bead portions exhibiting the large bulged deformation f issubjected to a higher tension to provide a higher apparent rigidity,while the carcass from the buttress portion to the radially outerportion of the sidewall exhibiting the small bulged deformation issubjected to a relatively low tension to provide a lower apparentrigidity. These facts can reduce the rolling resistance and, inaddition, improve the steering stability and riding comfortabilityagainst vibration.

First, a tire arranged to provide a slip angle will be considered. Inthis case, a lateral force acts upon the tire to cause traversedeformation. With the tire according to the invention, because of thehigh tension in the carcass in the proximity of the bead portions andhigh apparent rigidity, the tire exhibits a high rigidity against thetraverse deformation to provide a high cornering power and a highstability particularly in case of a large slip angle.

Next, a tire going over a protrusion on a road surface will beconsidered. In this case, the tire deforms much than when the tire issubjected to the normal load. Accordingly, absorption of the deformationby the sidewalls is important for improving the riding comfortabilityagainst vibration. It should be appreciated that the tire according tothe invention is constituted to lower the tension in the ply from theradially outer portion of the sidewall to the buttress portion, so thatthe apparent rigidity is lower to easily absorb the deformation, therebyimproving the riding comfortability against vibration.

As above described, the subject matter of the carcass radial surfaceprofile renders the carcass curvature reverse position as high aspossible. In order to provide such a profile after the tire is filledwith a normal inner pressure, it is necesssary to give a large bendingrigidity previously to the proximity of bead portions. However, thereinforcing the proximity of the bead portions by the use of reinforcingmembers more than reasonably required adversely affect the improvementor reduction of the rolling resistance which is the first object of theinvention. Accordingly, there is a difficulty in selection of structuralmembers to achieve the object of the invention. The inventors have foundthat the following two features are suitable for selections of theconstruction and members for achieving the object of the invention.

(1) A hard rubber having a hardness Hd of 80°-97° (Shore A hardness) isused for part or all of the bead filler rubbers of a tire.

(2) A height of the returning portion of a carcass extending radiallyoutwardly is equal to or higher than the height D of the carcasscurvature reverse position.

A bead filler rubber having a hardness less than 80° does not achievethe aimed effectiveness, while a bead filler rubber more than 97° servesto maintain the carcass radial surface profiles but adversely affectsthe durability of the tire. Using either or both of the above features(1) and (2) effectively exhibits the various performances of the tireaccording to the invention.

The feature of prohibiting the reinforcement of the proximity of thebead portions using reinforcing members much more reasonably requiredserves to easily provide the inwardly extending profile of the tire inthe zone from the tire maximum width position to the inner most positionas the curvature of the carcass reverses in the zone.

As above described, the tire according to the invention comprises atleast one layer of carcass consisting of a ply made of rubber coatedorganic fiber cords arranged substantially in radial surfaces of thetire and extending about bead wires and returning toward radiallyoutwardly of the tire, at least two layers of belts of rubber coatedcords having a high modulus of elasticity and intersecting each other atrelatively slight angles for reinforcing the tire in conjunction withthe carcass, and rubbers of sidewalls on both sides of the carcass and arubber of a tread outside the belt. Furthermore, it is required for thetire according to the invention to fulfil the following three conditions(a)-(c).

(a) While the tire mounted on a rim is being filled with the innerpressure at from 5% of the normal inner pressure to the normal innerpressure, the deformation in the zone from the tire maximum widthposition to the radially inner position is relatively large but thedeformation in the zone from the tire maximum width position towardradially outward is substantially little.

(b) When the tire mounted on the rim is filled with the normal innerpressure, the height of the carcass maximum width position from a rimbase is 50-60% of the height of the tire from the rim base to theoutermost surface of the tread.

(c) When the tire mounted on the rim is filled with the normal innerpressure, the height of the carcass curvature reverse position from therim base is 20-35% of the height of the tire from the rim base to theoutermost surface of the tread.

Such a tire fulfilling the above conditions, however, cannot bemanufactured by a tire vulcanizing mold having a normal configurationbecause of the particular shape of the tire intensionally deviating fromthe naturally equilibrated configuration. The inventors haveinvestigated this problem to find a tire vulcanizing mold having aconfiguration capable of manufacturing such a tire.

In general, a cavity of a tire vulcanizing mold is determined bymodifying somewhat a configuration corresponding to that of a tiremounted on a rim. According to this procedure, the heights of the moldmaximum width position and of the mold curvature reverse position aredetermined by making the tire vulcanizing mold itself commensurate withthe carcass radial surface profile of the tire to be manufactured. Onthe other hand, following the "strength of material" or "appliedmechanics" the inventors have studied the variation in the carcassradial surface profile when tires are being filled with inner pressureat from 5% to the normal inner pressure. As the result, the inventorshave found that such a variation in configuration arises into thedirection reducing its curvature in the zone of large curvature fromsidewalls to bead portions, while the variation arises into thedirection enlarging its curvature in the zone of small curvature andarises into the direction of plus R in the zone of reverse R,respectively.

For the above, it has been found that the variation in configurationarises into the direction deviating from the target carcass radialsurface profile. If the configuration of the vulcanizing mold formanufacturing a tire intentionally deviating from the naturallyequilibrated configuration is determined in consideration of this fact,the resultant mold configuration is in fact extremely unbalanced whichwould give rise to difficulties in manufacture and a tire manufacturedin the mold includes large stresses and strains therein when the tire ona rim is filled with the inner pressure, resulting in less durabilitywhich is unacceptable for practical use.

Distinct from the prior art, the configuration of the vulcanizing moldaccording to the invention is quite different from the carcass radialsurface profile of the tire mounted on a rim. Namely, the mold accordingto the invention has a cavity wherein a width of the cavity portioncorresponding to the rim for mounting the tire thereon is 20-50% wider(1-25" wider in case of 5" rim) than the width of the rim and the cavityprogressively widens from cavity portions corresponding to shoulders ofthe tire toward the cavity portion corresponding to the rim. The widthof the cavity corresponding to the width of the rim is referred hereinto as "foot width" which substantially corresponds to a distance betweenbead heels of the tire. Such a vulcanizing mold results from thesystematical investigation on the relation between the moldconfiguration and stress-strain conditions caused in tires on rimsfilled with inner pressure. When the tire manufactured in this mold ismounted on a rim narrower than the foot width of the mold the height ofthe maximum width position of the carcass radial surface profile ishigher than that of the maximum width position of the mold and theheight of the carcass curvature reverse position in the proximity of thebead portions is also higher than that of the maximum width position ofthe mold. When the foot width of the mold is less than the width of therim plus its 20%, the above effect cannot be accomplished. A foot widthof the mold more than the width of the rim plus its 50% is undesirablebecause mounting of the tire on the rim is difficult.

When the tire manufactured in the mold according to the inventionmounted on the rim is being filled with inner pressure at from 5% to thenormal pressure, the carcass radial surface profile varies in the mannerabove described. The carcass radial surface profile after filled withthe normal inner pressure is maintained in that intentionally deviatingfrom the naturally equilibrated configuration. It should be moreappreciated that with the mold according to the invention the variationis carcass radial surface profile during filling the inner pressure isin the direction returning to the carcass radial surface profile uponcompletion of vulcanizing, so that by mounting the tire on the rimnarrower than the foot width, the stress and strain once generated inthe tire are released without adversely affecting the durability of thetire.

FIG. 7 illustrates various profiles of cavities in molds A, B, C, D andE for tires of 185/70 SR14 with rims having a normal width 5". The moldsA and B have foot widths 1.5" and 2" wider than 5" rim width,respectively. The mold C has a foot width substantially equal to the 5"rim width. The molds D and E have foot widths 0.5" and 3" wider than 5"rim width, respectively. FIG. 8 illustrates the carcass radial surfaceprofiles of the tires manufactured in the above molds A, B, C, D and Ewhen the tires on the rims are filled with the normal inner pressure,the profiles A, B, C and D corresponding to the tires manufactured inthe molds A, B, C and D. It is evident that the profiles A and B deviatefrom the naturally equilibrated configuration and the profiles C and Dsubstantially coincide with the naturally equilibrated configurationwhich profiles C and D are not suitable for the purpose of theinvention. The tire manufactured in the mold E is not practically usedbecause of the wide distance between bead heels which makes it difficultto mount the tire on the rim. A curve P in FIG. 8 corresponds to a curveof comparative example 2 which will be explained later.

The effect of the tires constructed in the manner above describedaccording to the invention will be explained with reference toembodiments hereinafter.

Dimensions and properties of the tires of the invention and comparativeexamples are shown in Table 1. Each of carcasses 3 was of one plyincluding high modulus polyester cords of 1500d/2 arranged at 90° withrespect to an equatorial line of the tire. For each tire two belts 4were used, each including steel cords (twisted structure 1×5×0.25 mm)intersecting cords of the other belt and arranged at approximately 17°with respect to the equatorial line. The height F (FIG. 3a) of fillerswas 35% of the height H of the tires. These tires were mounted on 5Jrims. Other factors were equal in all the tires.

                                      TABLE 1                                     __________________________________________________________________________    Tires of the invention and comparative example                                                         Bead filler                                                                   Loss                                                                              Hardness                                                                            Loss modulus                               Tire   D/H                                                                              D/H                                                                              h/H                                                                              R/R'                                                                             f mm                                                                             f/D                                                                              tangent                                                                           (Shore A)                                                                           (dyn/cm.sup.2)                             __________________________________________________________________________    Comparative                                                                          46%                                                                              18%                                                                              47%                                                                              1.02                                                                             3.5                                                                              0  0.195                                                                             95    1.9 × 10.sup.8                       example 1                                                                     Comparative                                                                          67%                                                                              37%                                                                              47%                                                                              0.60                                                                             11.0                                                                             12 "   95    "                                          example 2                                                                     Invention 1                                                                          57%                                                                              24%                                                                              47%                                                                              0.80                                                                             6.0                                                                              7.5                                                                              "   95    "                                          Invention 2                                                                          62%                                                                              33%                                                                              47%                                                                              0.75                                                                             8.5                                                                              8  "   95    "                                          Invention 3                                                                          57%                                                                              22%                                                                              15%                                                                              0.82                                                                             5.5                                                                              6  "   95    "                                          Invention 4                                                                          57%                                                                              21%                                                                              47%                                                                              0.83                                                                             5.0                                                                              5.5                                                                              0.223                                                                             74    3.9 × 10.sup.7                       __________________________________________________________________________     Note:                                                                         Tire size: 185/70 SR14, Normal inner pressure: 1.7 kg/cm.sup.2, Rim:          5J14", Width of rim: 5" (127 mm)                                         

The carcass radial surface profiles of these tires are shown in FIG. 9.The profiles of the tires of invention 3 and 4 are substantially thesame as that of invention 1.

The rolling resistances of these tires are shown in Table 2, wherein therolling resistances of the tire of comparative example 1 are assumed 100as index numbers. The larger the index numbers, the better are rollingresistances.

In the test of rolling resistance, the tire was urged against a rotatingdrum having a 1707 mm diameter to rotatively drive the tire to apredetermined speed and whereupon a driving source for the drum is shutoff, so that although inertia forces of the drum and tire tend tocontinue their rotating movement, they progressively decrease theirrotating speeds because the kinetic energy is converted into heat energydue to repeated deformations at tire surfaces in contact with the drum,which heat energy dissipates into the atmosphere. The rollingresistances were calculated by degrees of deceleration during therotation of the drum and tire owing to their inertia forces.

                                      TABLE 2                                     __________________________________________________________________________    Rolling resistance of tire                                                              Tire                                                                Inner Speed                                                                             Comparative                                                                          Comparative                                                                          Invention                                                                          Invention                                                                          Invention                                                                          Invention                              pressure                                                                            km/H                                                                              example 1                                                                            example 2                                                                            1    2    3    4                                      __________________________________________________________________________    1.7 kg/cm.sup.2                                                                     50  100    102    109  111  106  106                                          80  100    101    112  116  106  106                                          100 100    102    118  120  108  106                                          150 100    100    122  126  110  108                                    2.1 kg/cm.sup.2                                                                     50  100    102    110  112  106  106                                          80  100    101    112  116  106  106                                          100 100    101    116  121  107  106                                          150 100    101    120  129  109  107                                    2.5 kg/cm.sup.2                                                                     50  100    100    110  113  106  106                                          80  100    100    112  116  107  106                                          100 100    101    116  120  108  106                                          150 100    101    120  130  110  108                                    __________________________________________________________________________     Note:                                                                         All the loads acting upon the tires for the test are the normal load for      1.7 kg/cm.sup.2 inner pressure according to JIS.                         

It can be seen from the Table 2, the tires 1 and 2 according to theinvention acheive the remarkable improvement or reduction of the rollingresistance, 10-30% as much as those of the tires of comparativeexamples. The tire 3 according to the invention is an example having aheight of the returning carcass lower than the carcass curvature reverseposition, whose effect is somewhat less than those of the tires 1 and 2according to the invention. The tire 4 according to the inventionincludes bead filler rubbers having a somewhat lower hardness, whichexhibits substantially the same effect as that of the tire 3.

From this viewpoint, the tires 1 and 2 according to the invention arethe most suitable in order to exhibit the effect of the invention to themaximum extent. However, even the tires 3 and 4 according to theinvention bring about the substantial effect of the invention to 6-10%improve or reduce the rolling resistance in comparison with thecomparative example 1.

In this manner, the tires according to the invention improve or reducethe rolling resistance by increasing the carcass maximum width positionand the carcass curvature reverse position, or enlarging within thesuitable values the curvature of carcass in the radially outer zone ofthe carcass maximum width position and simultaneously the carcasscurvature reverse extent in the radially inner zone of the sidewalls. Itis evident from the case of the comparative example 2 that no effect canbe accomplished if these values are too high extremely for the reasonabove described.

Table 3 shows cornering powers in comparison of the tire 1 according tothe invention with the tire 1 of comparative example, wherein thecornering power of the latter is assumed 100 as index number. The largerthe index number, the better is the steering performance. It can be seenfrom Table 3 that according to the invention the steering performancecan be improved as well as the rolling resistance.

                  TABLE 3                                                         ______________________________________                                        Comparison of steering performance                                                            Tire                                                                            Comparative                                                 Performance       example 1  Invention 1                                      ______________________________________                                        Cornering power kg/deg                                                                          100        105                                              ______________________________________                                    

Moreover, the drum for the test was formed on its cylindrical surfacewith protrusions which cause forces on the rotating axle of the tiresduring the rotation. The forces on the axle were measured to obtaincomparative data of the riding comfortability against vibration as shownin Table 4. The larger the index number in Table 4, the better is theriding comfortability against vibration. It should be understood thatthe tire according to the invention is superior in the ridingcomfortability against vibration and particularly improves thecomfortability greatly at high speeds from the data of reaction inhorizontal directions.

                  TABLE 4                                                         ______________________________________                                        Comparison of riding comfortability                                                                   Comparative                                           Measured item Condition example 1  Invention 1                                ______________________________________                                        Reaction in vertical                                                                        low speed 100        101                                        direction upon riding                                                                       high speed                                                                              100        102                                        over protrusion                                                               Reaction in horizontal                                                                      low speed 100        101                                        direction upon riding                                                                       high speed                                                                              100        111                                        over protrusion                                                               ______________________________________                                         Note:                                                                         Low speed: 20-50 km/H                                                         High speed: 60-120 km/H                                                  

Furthermore, the inventors compared wet performances of these tires onconcrete roads (skid number SN=35 indicating the roughness of the roads)and on asphalt roads (SN=50) to find that the tires according to theinvention are not inferior in the wet performance to the tires ofcomparative examples.

Moreover, the inventors compared distances over which these tirestraveled on the drum under high load and high inner pressure until theywere damaged. They found that the tires according to the invention arenot inferior in durability to those of the comparative examples.

As can be seen from the above description, the tires according to theinvention can remarkably improve or reduce the rolling resistancewithout adversely affecting wet performance and durability and alsoeffectively improve the steering stability and the riding comfortabilityagainst vibration. Furthermore, the method according to the inventionfacilitates the manufacture of tires having considerably improvedvarious performances.

It is further understood by those skilled in the art that the foregoingdescription is that of preferred embodiments of the disclosed tires andthat various changes and modifications may be made in the inventionwithout departing from the spirit and scope thereof.

What is claimed is:
 1. In a reduced rolling resistance pneumatic radialtire including a pair of annular sidewalls having at inner ends beadportions, respectively, a crown portion extending between radially outerends of said sidewalls, a carcass of at least one ply including organicfiber cord layers and having ends extending about bead wires embedded insaid bead portions with distal ends extending radially outwardly toreinforce said bead portions, said sidewalls and said crown portion, andhard rubber fillers filled in turns of said ply extending about saidbead wires, the improvement comprising said carcass having, on each sideof a plane including an equatorial line of the tire in a radialcross-section passing through a rotating axis of the tire, an outwardlyextending configuration with a center of a radius of curvaturepositioned in an inside of the carcass in a radially outer zone from acarcass maximum width position to said crown portion and further havingan inwardly extending configuration continuous with said outwardlyextending configuration with a center of a radius of curvaturepositioned in an outside of said carcass in a radially inner zone fromsaid carcass maximum width portion to said bead portion when said tiremounted on a rim is filled with an inner pressure 5% of a normal innerpressure; said outwardly extending configuration of said carcass in saidradially outer zone being maintained permitting a small change inconfiguration, while said inwardly extending configuration of saidcarcass in said radially inner zone being changed into a modifiedinwardly extending configuration with an increased radius of curvaturewhen said tire on the rim is filled with the normal inner pressure; anda height of said carcass maximum width portion from a bead base beingwithin a range of 50-65% of a height of the outermost portion of saidcrown portion from said bead base and a height of a junction of saidoutwardly and inwardly extending configurations from said bead basebeing within a range of 20-35% of said height of the outermost portionof said crown portion when said tire on the rim is filled with saidnormal inner pressure and equilibrated.
 2. A tire as set forth in claim1, wherein said change in configuration is within a range of 5-10% ofsaid height of the junction of said outwardly and inwardly extendingconfigurations when said inner pressure is increased to said normalinner pressure.
 3. A tire as set forth in claim 1 wherein each said hardrubber filler is tapered off from a position adjacent to upper portionof said bead wires to at least the junction of said outwardly andinwardly extending configurations.
 4. A tire as set forth in claim 3,wherein said distal end of said carcass extends beyond a radially outerend of said hard rubber filler but does not extend beyond said carcassmaximum width position.
 5. A tire as set forth in claim 1, wherein eachsaid hard rubber filler has a hardness 80°-97° in Shore A.
 6. A tire asset forth in claim 1, wherein each said hard rubber filler has a losstangent of 0.15-0.25 and a loss modulus of 8.0×10⁷ -2.5×10⁸ dyn/cm². 7.A tire as set forth in claim 2, wherein each said hard rubber filler istapered off from a position adjacent to upper portion of said bead wiresto at least the junction of said outwardly and inwardly extendingconfigurations.
 8. A tire as set forth in claim 7, wherein said distalend of said carcass extends beyond a radially outer end of said hardrubber filler but does not extend beyond said carcass maximum widthposition.
 9. A tire as set forth in claim 2, wherein each said hardrubber filler has a hardness 80°-97° in Shore A.
 10. A tire as set forthin claim 3, wherein each said hard rubber filler has a hardness 80°-97°in Shore A.
 11. A tire as set forth in claim 4, wherein each said hardrubber filler has a hardness 80°-97° in Shore A.
 12. A tire as set forthin claim 2, wherein each said hard rubber filler has a loss tangent of0.15-0.25 and a loss modulus of 8.0×10⁷ -2.5×10⁸ dyn/cm².
 13. A tire asset forth in claim 3, wherein each said hard rubber filler has a losstangent of 0.15-0.25 and a loss modulus of 8.0×10⁷ -2.5×10⁸ dyn/cm². 14.A tire as set forth in claim 4, wherein each said hard rubber filler hasa loss tangent of 0.15-0.25 and a loss modulus of 8.0×10⁷ -2.5×10⁸dyn/cm².
 15. A tire as set forth in claim 5, wherein each said hardrubber filler has a loss tangent of 0.15-0.25 and a loss modulus of8.0×10⁷ -2.5×10⁸ dyn/cm².
 16. In a reduced rolling resistance pneumaticradial tire including a pair of annular sidewalls having at inner endsbead portions, respectively, a crown portion extending between radiallyouter ends of said sidewalls, a carcass of at least one ply includingorganic fiber cord layers and having ends extending about bead wiresembedded in said bead portions with distal ends extending radiallyoutwardly, at least two belt layers consisting of rubber coated highmodulus cords arranged at comparatively small angles with respect to acentral circumferential line of the tire and intersecting with eachother encircling said carcass to reinforce therewith said sidewalls andsaid crown portion, and hard rubber fillers filled in turns of said plyextending about said bead wires, the imporvement comprising said carcasshaving, on each side of a plane including an equatorial line of the tirein a radial cross-section passing through a rotating axis of the tire, aradial surface profile so selected that a ratio of radii R/R' is withina range of 0.65-0.85 and f is within a range of 5-10 mm when said tiremounted on a normal rim is filled with a normal inner pressure where R'is a radius of a standard circle passing through points B, E and C,where the point B is an intersection of the carcass and a line extendingin parallel with said rotating axis and passing through a point at whicha flange of said rim starts leaving the tire, the point C is anintersection of the carcass and a line extending through the point B andperpendicular to said rotating axis of the tire and the point E is anintersection of a line extending through a middle point D of a linesegment BC and in parallel with said rotating axis and a line extendingthrough a carcass maximum width point F and perpendicular to therotating axis, and R is a radius of profile curvature of a shoulderpassing through said point C, and where f is the maximum distancebetween an arc of said standard circle and the remaining carcass linesmoothly, extending from said profile curvature of said shoulder to saidpoint B having a single carcass curvature reverse position.
 17. A tireas set forth in claim 16, wherein said profile curvature of saidshoulder passes through points F, I and C, where the point I is anintersection of the carcass and a line extending in parallel with therotating axis through a middle point H of a line segment GC, where apoint G is an intersection of the line segment BC and a line extendingthrough said carcass maximum width point F in parallel with saidrotating axis.